Generally, as shown in FIG. 1, a hermetic compressor comprises a motor 1 having a stator 2, a rotor 4 rotatably disposed around the stator 2, a crank shaft 6, a connecting rod 8, and a piston 10.
The crank shaft 6 has an lower eccentric portion 6a that is rotated by the rotor 4. One end of the connecting rod 8 has a great diameter 8a portion which is connected to the eccentric portion 6a, while the other end has a small diameter 8b portion which is connected to the piston 10. The piston 10 moves linearly in a piston chamber 12a of a cylinder 12. Referring to FIG. 2, the great diameter 8a portion has a great inner diameter hole h1 connected with the eccentric portion 6a, and the small diameter 8b portion has a small inner diameter hole h2 connected with a piston pin 11. The great inner diameter hole h1 and the small inner diameter hole h2 are connected by an oil transferring path 8c. 
In the above structure, the rotor 4 rotates the crank shaft 6 and the eccentric portion 6a. The connecting rod 8 converts the eccentric rotation of the eccentric portion 6a to a linear reciprocal motion of the piston 10. In other words, the reciprocal hermetic compressor is constructed to convert the rotation of the motor 1 to a linear movement of the piston 10.
In the meantime, the crank shaft 6 of the hermetic compressor forms a 90° angle with the connecting rod 8 when crossing the connecting rod 8. The 90° angle is an important element in the design of the compressor because it allows for maximum power transmitting efficiency. In order to ensure 90° angle, the permissible variations in the dimensions of related machine parts have to be known, and thus, it has been a general practice to disclose the tolerances of the related parts.
In addition, the piston 10 slides keeping an appropriate clearance in the piston chamber 12a in the cylinder 12, and also at a constant angle, i.e., 90°, with respect to the axis of the crank shaft 6.
It is important that the piston 10 forms a right angle with the piston pin 11. It is also important that axes of great inner diameter hole h1 and small inner diameter hole h2 are at a parallel relation with each other.
As described so far, various connecting relationships based on the connecting rod 8 have been described. In particular, the relationship between the connecting rod 8 and the crank shaft 6 affects the driving capability of the compressor. Accordingly, when the connecting rod 8 is misaligned or unstable with respect to the crank shaft 6, increased friction and abrasion between the elements is generated and the compressor may not operate normally. The problem is particularly aggravated when the crank shaft 6 and the piston 10 are not mechanically aligned.
Thus, there is a need to improve the reliability of the compressor by compensating for alignment errors in a connecting rod that converts the rotary motion of the crank shaft 6 to a linear motion in a piston.